In a method for producing a bonded wafer, as a method by which two wafers are bonded together and then one wafer is turned into a thin film, a method by which two wafers are bonded together and then one wafer is turned into a thin film by grinding/polishing and an ion implantation delamination method (also called the SmartCut (registered trademark) method) have been generally known.
The method by which two wafers are bonded together and then one wafer is turned into a thin film by grinding/polishing is specifically a method by which, for example, two silicon wafers are directly bonded or bonded with an oxide film sandwiched between them without using an adhesive, the bonding strength is increased by heat treatment (1000 to 1200° C.), and then one wafer is turned into a thin film by grinding/polishing. The advantages of this method are the crystallinity of an SOI layer and the reliability of a buried oxide film that are equal to those of an ordinary silicon wafer, and the drawbacks thereof are a film thickness uniformity limitation (about ±0.3 μm at most) of the SOI layer and high costs due to the use of two silicon wafers in the production of one SOI wafer.
On the other hand, the ion implantation delamination method is specifically a method by which, for example, after an ion implanted layer (a delaminating layer) is formed inside a wafer by implanting a gas ion of at least one of a hydrogen ion and a rare gas ion into one principal surface of at least one wafer (a bond wafer) of two silicon wafers, the ion implanted surface and one principal surface of the other silicon wafer (a base wafer) are brought into intimate contact with each other directly or through an oxide film, and the bond wafer is delaminated at the ion implanted layer by performing heat treatment at a temperature of 300° C. or higher. The ion implantation delamination method has advantages that, for example, a thin-film SOI wafer with SOI layer film thickness uniformity of ±10 nm or less can be made easily and cost reduction can be achieved by reusing the delaminated bond wafer more than once. However, due to the roughness of the surface of the SOI wafer immediately after delamination, the SOI wafer cannot be used directly as a substrate for fabricating a device, and an additional process for flattening is required.
As flattening processing, flattening by CMP (Chemical Mechanical Polishing), flattening by high-temperature heat treatment in an atmosphere of an inert gas, and flattening by heat treatment in an atmosphere of hydrogen or hydrogen chloride gas have been generally known. From a cost reduction standpoint, it has been considered that a method by which flattening heat treatment is performed in an atmosphere of hydrogen or hydrogen chloride is the most advantageous method (Patent Document 1).
The above-described flattening heat treatment in an atmosphere of hydrogen or hydrogen chloride will be described specifically. For example, when a bonded wafer is produced by the ion implantation delamination method by using a silicon single crystal wafer, as shown in FIG. 2, after a surface of a bond wafer 11, the surface in which an ion implanted layer 12 is formed, and a surface of a base wafer 13 are bonded together with an oxide film 14 sandwiched between them (FIG. 2(a)), the bond wafer 11 is delaminated at the ion implanted layer 12, whereby a bonded wafer 16 having a silicon thin film (an SOI layer) 15 on the base wafer 13 is made (FIG. 2(b)). Then, to reduce the roughness of the silicon thin film after delamination, heat treatment (hereinafter also referred to as gas etching) is performed on the bonded wafer 16 after delamination in an atmosphere containing hydrogen or hydrogen chloride by using a single wafer processing epitaxial layer growth reactor or the like (FIG. 2(c)).
However, this method is a method by which the SOI layer is flattened while being etched, and the film thickness distribution of the SOI layer exhibits lack of uniformity, and the SOI layer is thin on the periphery of the wafer and is thick in the central part thereof. As described above, when the thin film (the SOI layer) has poor film thickness uniformity, device characteristics vary, and therefore improvement of the film thickness uniformity of the bonded wafer is required.